This invention relates in general to electric switches and more specifically to switches that automatically open when the voltage across the switch drops below a predetermined level (an undervoltage condition). Switches of this type are commonly used as protective motor switches, that is, the switch prevents the operation of the motor unless the voltage applied to the motor is at or above a minimum operating level. Such switching devices are also useful when it is desired to prevent the uncontrolled start of machinery following a diminution or failure of electric power to the machinery. Heretofore such switching devices have commonly employed a bi-metallic element or a short circuit protection system to control a lock system that acts on a movable switching element to open the switch during an undervoltage condition. Such designs, however, have a complex construction, have elements which are difficult to fabricate, and are expensive to manufacture.
It is therefore a principal object of this invention to provide an electric switching device that has a reliable, relatively simple, and low cost undervoltage cut-out mechanism.